Solid electrolytic capacitor and process for producing the same

ABSTRACT

A capacitor element ( 10 ) fabricated by winding an anode foil ( 1 ) and a cathode foil ( 2 ) via a separator ( 3 ) is impregnated with a 3,4-ethylenedioxythiophene and an oxidizing agent to form poly(ethylenedioxythiophene) by chemical polymerization. A nonwoven fabric composed chiefly of a synthetic fiber is used as a separator, enabling a solid electrolyte to be favorably formed without being reacted with the oxidizing agent. Preferably, the capacitor element is dipped in water at 80 to 100° C. for 1 to 20 minutes to dissolve and remove the binder in the separator in order to preclude adverse effects on the electric characteristics caused by the binder. The oxidizing agent is used at a concentration in excess of 40% by weight with respect to the solvent, so that the degree of polymerization is high and a dense and homogeneous solid electrolytic layer is formed. To form the electrolytic layer, the capacitor element is impregnated with a monomer solution prepared by mixing 3,4-ethylenedioxythiophene and a volatile solvent at a volume ratio of 1:1 to 1:3. Then, the capacitor element is heat-treated and impregnated with a solution of oxidizing agent to form a dense and homogenous solid electrolytic layer.

TECHNICAL FIELD

The present invention relates to a process for producing a solidelectrolytic capacitor and more specifically, relates to a solidelectrolytic capacitor using a conducting polymer as the electrolyte.

BACKGROUND OF THE INVENTION

1. Conventional Materials for Solid Electrolytic Layer

Electrolytic capacitors comprise an oxide film layer functioning as adielectric material and an electrode drawn out from the oxide filmlayer, the oxide film layer being formed on an anode electrodecomprising a valve action metal such as tantalum and aluminium and beingarranged with micro-pores and etching pits.

Herein, the electrode is drawn out, via an electrolytic layer withelectric conductivity, from the oxide film layer. Accordingly, theelectrolytic layer serves as a practical cathode in such electrolyticcapacitors. For an aluminium electrolytic capacitor, for example, aliquid electrolyte is used as a practical electrode, while the cathodeelectrode only serves for the electrical connection between the liquidelectrolyte and an external terminal.

The electrolytic layer functioning as a practical cathode shouldessentially be adhesive to the oxide film layer, and be dense anduniform. Specifically, the adhesion inside the micro-pores and etchingpits of the anode electrode significantly influences the electricalperformance. Conventionally, therefore, numerous electrolytic layershave been proposed.

Solid electrolytic capacitors comprise solid electrolytes with electricconductivity, in place of liquid electrolytes defective of any impedencecharacteristic in the high-frequency region due to the ion conductivity.Specifically, manganese dioxide and 7, 7, 8, 8-tetracyanoquinodimethane(TCNQ) complex have been known as such solid electrolytes.

More specifically, a solid electrolytic layer comprising manganesedioxide is produced by dipping an anode element comprising sinteredtantalum in an aqueous manganese nitrate solution, followed by thermaldecomposition at a temperature around 300° C. to 400° C. The oxide filmlayer in capacitors comprising such solid electrolytic layer is readilydamaged during the thermal decomposition of manganese dioxide, so theleakage current is likely to increase; because the specific resistanceof manganese dioxide is high, additionally, the resulting impedencecharacteristic is not sufficiently satisfactory.

Furthermore, the lead wire is damaged at the thermal process. Therefore,a post-process is needed to additionally arrange an outer connectingterminal.

Alternatively, a solid electrolytic capacitor comprising the TCNQcomplex as described in Japanese Patent Laid-open No. 58-191414 has beenknown as one of the aforementioned solid electrolytic capacitors, whichis produced by thermally melting the TCNQ complex, and dipping an anodeelectrode in the resulting melt TCNQ complex or coating the resultingmelt TCNQ complex on the anode electrode. The TCNQ complex is highlyconductive, with the resultant great effects in terms of frequencycharacteristic and temperature performance.

Because the melting point of the TCNQ complex and the decompositionpoint thereof are very close so the melt TCNQ complex is readilyconverted to an insulating substance under some temperature condition,the temperature control of the complex is tough during the process ofcapacitor production; additionally because the TCNQ complex per se isdefective of thermal resistance. the characteristic properties of thecomplex are distinctively modified by the soldering heat during themounting process on a print board.

2. Application of Conducting Polymer

So as to overcome the inconvenience of manganese dioxide and TCNQcomplex, furthermore, attempts have been made in recent years about theuse of conducting polymers such as polypyrrole as solid electrolyticlayer.

Conducting polymers typically including polypyrrole are primarilyproduced by chemical oxidation polymerization (chemical polymerization)and electrolytic oxidation polymerization (electrolytic polymerization).It has been difficult to produce a dense layer with a large strength bychemical polymerization.

By electrolytic polymerization, alternatively, a voltage should beapplied to a subject material on which an oxide film layer is to beformed. Therefore, it is difficult to apply electrolytic polymerizationto an anode electrode with an insulating oxide film layer formed on thesurface thereof for electrolytic capacitors. Hence, a process has beenproposed, comprising preliminarily forming a conductive precoatinglayer, for example a conducting polymer layer formed by chemicalpolymerization using an oxidant, on the surface of an oxide film layer,and subsequently forming an electrolytic layer by electrolyticpolymerization using the precoating layer as an electrode (JapanesePatent Laid-open 63-173313, Japanese Patent Laid-open 63-158829;manganese dioxide functions as the precoating layer).

However, the process of preliminarily forming the precoating layer iscomplicated; and by electrolytic polymerization, a solid electrolyticlayer is formed, starting from the proximity of an outer electrodearranged on the positive electrode face covered with the oxide filmlayer for the purpose of polymerization. Accordingly, it has been verydifficult to continuously form a conducting polymer film of a uniformthickness over a wide range.

Thus, another attempt has been made to form an electrolytic layercomprising a conducting polymer film, by winding an anode electrode anda cathode electrode, both in foil shapes, while a separator isinterposed between these electrodes, to form a so-called wound capacitorelement, allowing the capacitor element to be impregnated with a monomersuch as pyrrole and an oxidant, to form the conducting polymer film bychemical polymerization alone.

Such wound capacitor element has been known for aluminium electrolyticcapacitors. It has been desired to avoid any complicated electrolyticpolymerization by supporting the conducting polymer layer with aseparator and to enlarge the capacity of the resulting capacitor byusing an electrode in a foil shape of a larger surface area.

Both the electrodes and the separator can be supported at a constantfastening strength by using the wound capacitor element, which isindicated to make contribution to the adhesion between both theelectrodes and the electrolytic layer.

When the capacitor element is impregnated with a mixture solution of themonomer and an oxidant, the monomer and the oxidant are rapidlypolymerized together, so that the resulting solid electrolytic layer isnever formed deeply inside the capacitor element. Thus, it has beenfound that the desired electrical performance can never be yielded.

Then, an attempt has been made to lower the polymerization temperatureof the solution during the polymerization reaction, with the resultantmore or less great electrical performance. Nevertheless, the resultingpressure resistance is still insufficient, disadvantageously.

Additionally, chemical polymerization at low temperature requires stricttemperature control and a complicated apparatus, so that the finalproduct is disadvantageously costly.

3. Poly(ethylenedioxythiophene) of Interest

Alternatively, various conducting polymers have been examined. Atechnique (Japanese Patent Laid-open 2-15611) focused onpoly(ethylenedioxythiophene) (PEDT) at a slow reaction velocity and withexcellent adhesion to the oxide film layer of the anode electrode hasbeen reported.

With attention focused on the slow polymerization velocity ofpoly(ethylenedioxythiophene), the present inventors have submitted anapplication (Japanese Patent application 8-131374) of an invention togenerate poly(ethylenedioxythiophene) inside a capacitor element,comprising winding through the medium of a separator an anode electrodefoil and a cathode electrode foil to fabricate a capacitor element,allowing the capacitor element to be impregnated with a mixture solutionof a monomer and an oxidant solution, and generating a solid electrolytepoly(ethylenedioxythiophene) by the chemical polymerization of themonomer and the oxidant. The polymerization proceeded slowly.

4. Problems that the Invention is to Solve

A solid electrolytic capacitor produced by allowing a capacitor elementto be impregnated with a mixture solution of a monomer and an oxidant byusing a separator for use in general electrolytic capacitors to generatepoly(ethylenedioxythiophene), never exerts satisfactory ESR performance;and additionally, the static capacity and life of the resulting solidelectrolytic capacitor are at large variations. This is possibly due tothe facts that the use of such general separators is inconvenient forthe generation of poly(ethylenedioxythiophene) and that the conditionsfor allowing the capacitor element to be impregnated with a monomer andan oxidant are not satisfactory. The finding is now described in moredetail below.

Because an oxidant ferric p-toluenesulfonate is used for the generationof poly(ethylenedioxythiophene), separators composed of manila paper foruse in general electrolytic capacitors induce a chemical reaction,damaging the oxidative action of the oxidant and additionally causing anaccident such as short circuit due to the separator damage.

On contrast, glass paper and the like are potentially useable for theseparator, but glass paper of general thickness of 80 to 200 μm ishardly slimmed approximately to the thickness of manila paper separatorof 40 μm; and because the folding strength is more or less small, asmall-size product is hardly produced. Because glass paper is nothydrophilic, a conductive dense and uniform polymer layer, namely solidelectrolytic layer, is hardly formed, which possibly affects theelectrical performance of the resulting capacitor, disadvantageously.

Additionally, simple impregnation with a mixture solution of a monomerand an oxidant solution does not yield a polymer at a satisfactorypolymerization degree, so that a sufficiently dense and uniform solidelectrolytic layer is hardly formed inside the resulting capacitorelement. During the impregnation with a mixture solution of a monomerand an oxidant solution, in particular, the polymerization reaction ofthe mixture solution progresses over time, so that the capacitor elementis impregnated with the mixture solution, in the course of thepolymerization reaction. Thus, the mixture solution is solidifiedintermediately on the way of the permeation of the mixture solutioninside the capacitor element, whereby the resulting solid electrolyticlayer is likely to be non-uniform. So as to permeate the mixturesolution further inside the capacitor element in order to compensatesuch intermediate solidification of the mixture solution, the capacitorelement should continuously be impregnated with the mixture solution.However, such continuous impregnation of the mixture solution costsneedless materials and a longer time, with the resultant decrease of theproductivity.

5. Objects of the Invention

The present invention has been proposed so as to overcome the problems.An object resides in the production of a solid electrolytic layercomprising a dense and uniform conducting polymer inside a woundcapacitor element, by modifying the separator for use in the capacitorelement and the impregnation conditions of the capacitor element with amonomer and an oxidant, to provide a solid electrolytic capacitor withexcellent electrical performance and a large capacity. Additionally, theother object is to provide a process for producing such great solidelectrolytic capacitor at a high efficiency and a high productivity.

DISCLOSURE OF THE INVENTION

So as to achieve the objects, in accordance with the invention, amodified solid electrolytic capacitor is provided, which is produced bywinding through the medium of a separator an anode electrode foil and acathode electrode foil to fabricate a capacitor element, allowing thecapacitor element to be impregnated with 3,4-ethylenedioxythiophene andan oxidant to generate poly(ethylenedioxythiophene) by chemicalpolymerization; and a process for producing such solid electrolyticcapacitor is also provided.

In accordance with the invention, a solid electrolytic capacitor with amodified separator comprising a nonwoven fabric composed chiefly of asynthetic fiber is provided. The nonwoven fabric is preferably vinylonfiber or vinylon fiber mixed with glass fiber, polyester fiber, nylonfiber, rayon fiber or paper fiber. Since the separator composed chieflyof such synthetic fiber never reacts with any oxidant and is misciblewith solvents, the monomer and an oxidant can readily permeate insidethe wound capacitor element, so that a dense and uniform solidelectrolytic layer can be yielded. Additionally, such separator isthinner and more flexible than a glass paper of a thickness of 80 to 200μm, so that the quantities of the wound electrode foils and theseparator are increased per each unit volume.

It has been found that the separator using the nonwoven fabric composedchiefly of such synthetic fiber can hardly yield the desired staticcapacity or thermal resistance. The reason is not yet found, but thebinder in the nonwoven fabric may possibly have some influence. Based onthe possibility, in accordance with the invention, a process forproducing a solid electrolytic capacitor with a modified separator isprovided, comprising dipping the wound capacitor element in water at 80°C. to 100° C. for one to 10 minutes to dissolve the binder in theseparator in water and then discard the binder and allowing theresulting separator to be impregnated with 3,4-ethylenedioxythiopheneand in oxidant. The process may comprise dissolving and discarding thebinder in the separator, drying the capacitor element at 80° C. to 100°C., and thereafter allowing the capacitor element to be impregnated with3,4-ethylenedioxythiophene and an oxidant. More preferably, a series ofprocesses, namely the process of discarding the binder in water and thesubsequent drying process, is repeated at least two times.

In accordance with the invention, additionally, a process of modifyingthe conditions for the impregnation with the monomer and an oxidant isprovided, comprising the impregnation with 3,4-ethylenedioxythiopheneand an oxidant at a concentration above 40% by weight to a solvent. Thesolvent is preferably butanol, while the oxidant is preferably selectedfrom the group consisting of p-toluenesulfonic acid, ferricdodecylbenzenesulfonate and ferric chloride. Sufficient ESRcharacteristic cannot be recovered when an oxidant is used at aconcentration below 40% by weight to a solvent. However, the ESRcharacteristic can be improved, distinctively, by using an oxidant at aconcentration above 40% by weight to a solvent.

In accordance with the invention, still furthermore, a process ofmodifying the conditions for the impregnation with the monomer and anoxidant is provided, comprising allowing the capacitor element to beimpregnated with the monomer 3,4-ethylenedioxythiophene and subsequentlyallowing the resulting capacitor element to be impregnated with anoxidant. Because the monomer for primary impregnation and distributionthereafter inside the capacitor element can be polymerized chemicallywith the oxidant for subsequent impregnation inside the capacitorelement, a dense and uniform solid electrolytic layer can be generatedinside the wound capacitor element according to the process. Morepreferably, the monomer is diluted with a volatile solvent and is thenthermally treated; and then, an oxidant solution is used for theimpregnation. Because the diluted monomer is used for the uniformimpregnation of the capacitor element and the volatile solvent thereincan be vaporized by the subsequent thermal process, in this case, asolid electrolytic layer of higher quality can be produced. Additionallybecause the thermal process requires only a short time, the productivityis also great.

The volatile solvent for use according to the process is preferablyselected from the group consisting of hydrocarbons, ethers, esters,ketones, alcohols and nitrogen compounds. These materials aresatisfactorily miscible with the monomer to promote the uniformimpregnation with the monomer, with no disadvantageous effect on theelectrode foils comprising for example aluminium. Methanol, ethanol andacetone are more preferable owing to the economy and readyhandleability. At the process of allowing the capacitor element to beimpregnated with a monomer solution, furthermore, the monomer solutionis first prepared by mixing together a monomer and a volatile solvent,but the composition thereof possibly changes over time due to theevaporation of the volatile solvent after the mixing process. Hence, themonomer and the volatile solvent are preferably used separately for theimpregnation. In this case, the capacitor element can be impregnatedwith a monomer solution with less compositional change. By allowing thecapacitor element to be impregnated with a monomer solution at a ratioof the monomer and a volatile solvent being 1:1 to 1:3, the capacitorelement can be uniformly impregnated with the monomer, with theresultant high-quality solid electrolytic layer. Because the volatilesolvent is used at the lowest limit as needed in this case, furthermore,the productivity is never lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a decomposition perspective view of one example of the solidelectrolytic capacitor produced in accordance with the invention; and

FIG. 2 is an enlarged cross sectional view depicting an anode electrodefoil with the solid electrolytic layer formed in accordance with theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Best mode for carrying out the invention is specifically described withreference to drawings.

1. Essential Production Process

FIG. 1 depicts the solid electrolytic capacitor produced in accordancewith the invention, which is essentially produced by the followingprocedures.

First, capacitor element 10 is fabricated by winding anode electrodefoil (positive electrode foil) 1 comprising a valve action metal such asaluminium with an oxide film layer formed on the surface thereof andcathode electrode foil (negative electrode foil) 2 through the medium ofseparator 3. Then, the capacitor element 10 is impregnated with3,4-ethylenedioxythiophene and an oxidant; through chemicalpolymerization in the capacitor element 10, solid electrolytic layer 5comprising poly(ethylenedioxythiophene) is formed. The solidelectrolytic layer 5 is supported with the separator 3.

More specifically, the positive electrode foil 1 comprises a valveaction metal such as aluminium; as shown in FIG. 2, the surface thereofis roughened by electro-chemical etching process in an aqueous chloridesolution, to form numerous etching pits 8, 8, - - - . Furthermore, avoltage is applied to the surface of the positive electrode foil 1 in anaqueous solution of for example ammonium borate, to form oxide filmlayer 4 functioning as a dielectric material. Like the positiveelectrode foil 1, the negative electrode foil 2 comprises aluminium andthe like; and the surface thereof is simply treated with etchingprocess.

The positive electrode foil 1 and the negative electrode foil 2 areseparately connected with lead wires 6,7 by known means such as stitchor ultrasonic welding, to connect these electrodes outside. The leadwires 6, 7 comprising aluminium and the like compose outer connectionparts responsible for the electrical connection of the positiveelectrode foil 1 and the negative electrode foil 2 to the outside. Thelead wires 6, 7 are drawn out from the end face of the wound capacitorelement 10.

The dimensions of both the electrode foils 1, 2 are appropriatelydetermined, depending on the specification of a designed solidelectrolytic capacitor; and the separator 3 of a slightly larger widththan the widths of both the electrode foils 1, 2 is satisfactorily used.

Herein, 3,4-ethylenedioxythiophene can be prepared by the known processdisclosed in Japanese Patent Laid-open 2-15611 and the like.Additionally, ferric p-toluenesulfonate dissolved in butanol is used asthe oxidant.

2. Solid Electrolytic Capacitor with Modified Separator

One embodiment of the modified separator in accordance with theinvention comprises a nonwoven fabric composed chiefly of vinylon fiber;another embodiment thereof comprises a nonwoven fabric composed of amixture of vinylon fiber with glass fiber, polyester fiber, nylon fiber,rayon fiber or paper fiber such as manila paper. More specifically, thenonwoven fabric is at a unit weight of 6 to 36 g/m2, a fiber diameter of5 to 30 μm, a thickness of 30 to 150 μm and a density of 0.2 to 0.5g/cm3.

Because the resulting modified separators never chemically react withthe oxidant and are miscible with the solvent for the oxidant, themodified separators can improve the efficiency of the resultingcapacitor element per unit volume, despite the thickness approximatelyequal to the thickness (40 μm) of manila paper separators for use inconventional electrolytic capacitors, with no deterioration of thepermeability of the monomer and oxidant for use in the impregnation.Accordingly, the resulting solid electrolytic capacitor can bedown-sized and have a larger capacity.

The solid electrolytic capacitor with such modified separator can beproduced, by forming a wound capacitor element using the separator,dipping the capacitor element in water at 80° C. to 100° C. for one to10 minutes to dissolve and remove the binder in the separator, dryingthe resulting capacitor element at 80° C. to 120° C., and subsequentlyallowing the capacitor element to be impregnated with3,4-ethylenedioxythiophene and an oxidant.

Because the binder in the nonwoven fabric composing the separator can bedissolved and removed by the process, the reduction of the staticcapacity due to the presence of the binder can be prevented.Accordingly, the capacitor performance can be improved.

A specific process for producing the solid electrolytic capacitor withthe modified separator is now specifically described below, along withexamples of the solid electrolytic capacitor produced by the process,compared with comparative examples.

2.1 Comparison Between Modified Separator and Glass Separator

First, comparison will be made between the solid electrolytic capacitorwith the modified separator (Example 1) in accordance with the inventionand a solid electrolytic capacitor with a separator composed of glasspaper (Comparative Example 1).

EXAMPLE 1

The positive electrode foil 1 and the negative electrode foil 2 arewound through the medium of separator 3 comprising a nonwoven fabriccomposed chiefly of a vinylon fiber at a 50-μm thickness and a unitweight of 12 g/m2, to fabricate capacitor element 10 as shown in FIG. 1.Herein, the capacitor element 10 is at a diameter of 4 φ and alongitudinal length of 7 mm and with a nominal voltage of 16 WV and anominal static capacity of 10 μF.

By dipping the capacitor element 10 in water at 100° C. for 5 minutes,the binder in the separator 3 is dissolved therein and is then removed.If necessary, the step may satisfactorily be repeated several times at agiven interval.

Then, the capacitor element 10 is impregnated with3,4-ethylenedioxythiophene and an oxidant. As descried above, herein,ferric p-toluenesulfonate dissolved in butanol is used as the oxidant.Additionally, the blend ratio of 3,4-ethylenedioxythiophene and theoxidant is for example 1:5.

By leaving the capacitor element 10 impregnated with3,4-ethylenedioxythiophene and the oxidant to stand alone forpolymerization at a polymerization temperature of 25° C. to 150° C. fora duration of 2 to 15 hours, solid electrolytic layer 5 comprisingpoly(ethylenedioxythiophene) can be produced.

As to the ranges of the polymerization temperature and the durationduring which the capacitor element 10 is left to stand, a higherpolymerization temperature is likely to deteriorate the leakage currentcharacteristic although the static capacity, tan δ and the impedencecharacteristic are improved, among the electrical properties of theresulting solid electrolytic capacitor. Thus, the polymerizationtemperature can appropriately be modified within the range, depending onthe specification of the designed capacitor element 10.

At the aforementioned steps, the solid electrolytic layer 5 is formed onthe separator 3 interposed between the positive electrode foil 1 and thenegative electrode foil 2; by subsequently covering the exterior surfaceof the resulting capacitor element 10 with an exterior resin, a solidelectrolytic capacitor can be recovered.

Comparative Example 1

The positive electrode foil 1 and the negative electrode foil 2 arewound through the medium of a separator composed of a glass paper at a150-μm thickness and a unit weight of 20 g/m2, to fabricate a capacitorelement as shown in FIG. 1. Under the same conditions as in Example 1,the capacitor element is impregnated with 3,4-ethylenedioxythiophene andthe oxidant, to forme a solid electrolytic layer comprisingpoly(ethylenedioxythiophene), followed by covering with an exteriorresin, to recover a solid electrolytic capacitor with a nominal voltageof 16 WV and a nominal static capacity of 10 μF.

Comparison of Initial Performances

The initial performances of the solid electrolytic capacitors of Example1 and Comparative Example 1 were determined. The results are shown belowin Table 1. Herein, the term capacity occurrence ratio means the ratioof the determined static capacity to the nominal static capacity.

TABLE 1 Static Capacity Leakage capacity occurrence tan δ ESR current120 Hz ratio 120 Hz 100 KHz 20 V (μF) (%) (%) (Ω) (μA) Comparative 8.686.0 1.8 0.065 0.23 Example 1 Example 1 9.4 94.0 1.8 0.070 0.15

As apparently shown in Table 1, the solid electrolytic capacitor ofExample 1 wherein the 50-μm thick separator comprising the nonwovenfabric composed chiefly of the vinylon fiber can exert the properties atalmost the same levels of those of Comparative Example 1 using a 150-μmthick separator composed of the glass paper. Therefore, the capacityefficiency of Example 1 using the separator of a thickness ⅓-fold thatof the separator of Comparative Example 1 is apparently improved farbetter than Comparative Example 1.

2.2 Comparison of Binder Effect

EXAMPLE 2

In the same manner as in Example 1, the positive electrode foil 1 andthe negative electrode foil 2 are wound through the medium of separator3 comprising a nonwoven fabric composed chiefly of a vinylon fiber at a50-μm thickness and a unit weight of 12 g/m2, to fabricate capacitorelement 10. After dipping the resulting capacitor element 10 in water at100° C. for 5 minutes to dissolve and remove the binder in theseparator, the capacitor element 10 is dried at 100° C. for 10 minutes.

Under the same conditions as in Example 1, the capacitor element 10 isimpregnated with 3,4-ethylenedioxythiophene and an oxidant, to form asolid electrolytic layer comprising poly(ethylenedioxythiophene),followed by covering with an exterior resin, to produce a solidelectrolytic capacitor at a nominal voltage of 6.3 WV and a nominalstatic capacity of 33 μF.

EXAMPLE 3

The capacitor element 10 fabricated in the same manner as in Example 2is subjected two times in a repetitious manner to a series of steps,namely a step of dissolving and removing the binder in the separator bydipping the capacitor element 10 in water at 100° C. and a step ofdrying the resulting capacitor element 10 at 100° C. for 10 minutes.Under the same conditions as in Example 1, the capacitor element 10 isimpregnated with 3,4-ethylenedioxythiophene and the oxidant, to form asolid electrolytic layer comprising poly(ethylenedioxythiophene),followed by covering with an exterior resin, to recover a solidelectrolytic capacitor at a nominal voltage of 6.3 VW and a nominalstatic capacity of 33 μF.

Comparative Example 2

In the same manner as in Example 1, the positive electrode foil 1 andthe negative electrode foil 2 are wound through the medium of separator3 comprising a nonwoven fabric composed chiefly of a vinylon fiber at a50-μm thickness and a unit weight of 12 g/m2, to fabricate capacitorelement 10. By dipping the capacitor element 10 with3,4-ethylenedioxythiophene and an oxidant under the same conditions asin Example 1, with no step of dissolving and removing the binder inwater or no subsequent drying step, a solid electrolytic capacitor at anominal voltage of 6.3 WV and a nominal static capacity of 33 μF isproduced.

Comparison of Initial Performances

The initial performances of the solid electrolytic capacitors ofExamples 2 and 3 and Comparative Example 1 were determined. The resultsare shown below in Table 2. As described above, herein, the termcapacity occurrence ratio means the ratio of the determined staticcapacity to the nominal static capacity.

TABLE 2 Static Capacity Leakage capacity occurrence tan δ ESR current120 ratio 120 Hz 100 KHz 6.3 V Hz (μF) (%) (%) (Ω) (μA) Comparative 28.185.2 6.1 0.035 5.10 Example 2 Example 2 31.5 95.5 5.1 0.045 4.45 Example3 32.8 99.4 6.2 0.037 1.50

As apparently shown in Table 2, the capacity occurrence ratios of thesolid electrolytic capacitors of Examples 2 and 3 with the binderremoval process are larger than the ratio of the solid electrolyticcapacitor of Comparative Example 2 with no binder removal process,suggesting that the effect of the binder in the nonwoven fabric composedof synthetic fiber is reduced. As described above, more specifically,the removal of the binder in the separator comprising the nonwovenfabric composed chiefly of synthetic fiber may possibly bring thedesired static capacity and thermal resistance, which can hardly berecovered by such separator unless the binder is removed from theseparator.

As apparent on comparison between Examples 2 and 3, furthermore, thecapacity occurrence ratio in Example 3 is larger than the ratio inExample 2, because a series of steps, namely the step of dissolving andremoving the binder in water and the subsequent drying step, is repeatedtwo times in Example 3 but is carried out only once in Example 2. Thus,it is indicated that two or more repetitions of the series of thesesteps can reduce the binder effect.

2.3 Modification Example

The process using the modified separator is not specifically limited tothe aforementioned process but can be modified appropriately. Thenonwoven fabric composing the separator is not limited to those inExamples 1 to 3 but includes appropriate types of nonwoven fabricscomposed chiefly of synthetic fiber. From the respects of down-sizingand capacity increase, the separator is preferably at a thickness ofabout 50 μm or less, whereby a capacitor at a dimension and nominalvalues nearly equal to the dimension and nominal values, respectively ofa conventional separator composed of manila paper, can be recovered.

3. Process Using Oxidant at Modified Concentration

In one embodiment of the inventive process, ferric p-toluenesulfonatedissolved in a solvent butanol is used as the oxidant. It is found thatthe oxidant at a concentration above 40% by weight to butanol can yieldgreat results in this case. The reason is not yet elucidated, but theoxidant at a high concentration promotes chemical polymerization toelevate the polymerization degree, so that the electric conductivity ofthe resulting solid electrolytic layer can be improved.

The oxidant is blended at a concentration above 40% by weight inbutanol, because sufficient static capacity or ESR characteristic cannever be recovered below 40% by weight. Furthermore, the substantialupper limit is about 60% by weight. Synthetic reaction with the oxidantabove the limit hardly proceeds. Therefore, the oxidant is blended at aconcentration within a range of 50% by weight to 55% by weight, from thestandpoints so as to recover desired properties and readily proceed thesynthesis.

The ratio of butanol and ferric p-toluenesulfonate in the oxidantsolution is appropriately determined, but preferably, the ratio is 1:3to 1:15.

The process for producing the solid electrolytic capacitor using theoxidant at such modified concentration is now specifically describedbelow, along with examples of the solid electrolytic capacitor producedby the process, compared with comparative examples.

3.1 Production Process

The positive electrode foil 1 and the negative electrode foil 2 arewound through the medium of separator 3 comprising a nonwoven fabriccomposed chiefly of a vinylon fiber, to fabricate capacitor element 10as shown in FIG. 1. Herein, the capacitor element 10 is at a diameter of4 φ and a longitudinal length of 7 mm and with a nominal voltage of 16WV and a nominal static capacity of 10 μF.

Then, the capacitor element 10 is impregnated with3,4-ethylenedioxythiophene and an oxidant, to form solid electrolyticlayer 5 comprising poly(ethylenedioxythiophene). The oxidant comprisesferric p-toluenesulfonate dissolved at six different levels ofconcentrations (40% by weight in Comparative Example 3; 44% by weight inExample 4; 48% by weight in Example 5; 52% by weight in Example 6; 56%by weight in Example 7; and 60% by weight in Example 8) in butanol. Theblend ratio of 3,4-ethylenedioxythiophene and the oxidant is 1:5.

At the aforementioned process, solid electrolytic layer 5 is formed onseparator 3 interposed between the positive electrode foil 1 and thenegative electrode foil 2; subsequently, the exterior surface of thecapacitor element 10 is covered with an exterior resin, to recover asolid electrolytic capacitor.

3.2 Change of Properties Due to Oxidant Concentration

The six types of the solid electrolytic capacitors (Examples 4 to 8 andComparative Example 3) produced by using the oxidant at differentconcentrations were examined of the change of their properties due tothe difference in oxidant concentration. The results are shown below inTable 3.

As apparently shown in Table 3, no satisfactory ESR characteristic canbe procured in Comparative Example 3 using the oxidant ferricp-toluenesulfonate dissolved at 40% by weight in the solvent, while thecapacity occurrence ratio in Comparative Example 3 is 30.6 μ F, whichcorresponds to about 93% of the nominal static capacity (33 μF). The ESRcharacteristic is dramatically improved in Examples 4 to 8 using theoxidant at a concentration above 40% by weight, which indicates that adense and uniform solid electrolyte is formed inside the capacitorelement.

TABLE 3 Static capacity ESR Oxidant 120 Hz 100 KHz concentration (μF)(Ω) Comparative 40 wt % 30.6 0.079 Example 3 Example 4 44 wt % 32.20.039 Example 5 48 wt % 33.0 0.029 Example 6 52 wt % 33.4 0.024 Example7 56 wt % 33.5 0.025 Example 8 60 wt % 32.8 0.024

3.3 Modification Example

The process using the oxidant at the modified concentration is notspecifically limited to the aforementioned process but can be modifiedappropriately. The concentration of the oxidant is not specificallylimited to the concentrations in Examples 4 to 8. The concentration canbe appropriately selected from concentrations above 40% by weight to thesolvent.

4. Process Including Process of Forming Modified Solid ElectrolyticLayer

In one embodiment of the inventive process, a solid electrolytic layeris produced by allowing a capacitor element to be impregnated with amonomer solution of a mixture of 3,4-ethylenedioxythiophene and avolatile solvent, subjecting the resulting capacitor element to thermaltreatment and subsequently allowing the capacitor element to beimpregnated with an oxidant solution.

Known means, for example impregnation under reduced pressure andimpregnation under pressure, can be applied to the process of allowingcapacitor element 10 to be impregnated with a monomer solution of amixture of 3,4-ethylenedioxythiophene and a volatile solvent. Thesemeans can also be applied to the process of allowing the capacitorelement after thermal treatment to be impregnated with an oxidantsolution.

As the volatile solvent, additionally, use is made of a materialselected from hydrocarbons, ethers, esters, ketones, alcohols andnitrogen compounds. More specifically, pentane and hexane and the likecan be used as the hydrocarbons; tetrahydrofuran and dipropyl ether andthe like can be used as the ethers; methyl formate and ethyl acetate andthe like can be used as the esters; acetone and methyl ether ketone andthe like can be used as the ketones. Furthermore, methanol, ethanol andpropanol and the like can be used as the alcohols; and acetonitrile andthe like can be used as the nitrogen compounds.

Among the volatile materials described above, methanol, ethanol andacetone and the like, in particular, are preferably used as describedabove. Because water is slightly soluble in 3,4-ethylenedioxythiophene,furthermore, water is not preferably used as the volatile solvent.

The following advantages can be brought about by the aforementionedprocess.

Because the quantity of the monomer 3,4-ethylenedioxythiophene is farless than the amount of the oxidant, the monomer is readily distributednon-uniformly in the capacitor element 10 when the capacitor element 10is impregnated with the monomer alone; according to the process,however, the monomer is diluted with volatile solvents such as methanol,ethanol and acetone, whereby the capacitor element 10 is uniformlyimpregnated with the monomer. In this case, the capacitor element 10 maysatisfactorily be impregnated with a monomer solution of a mixture ofthe monomer and such volatile solvent. According to the process,alternatively, the monomer and the volatile solvent may satisfactorilybe used separately for the impregnation of the capacitor element 10,whereby the capacitor element can be impregnated with a monomer solutionwith less compositional change over time.

By thermally treating the capacitor element 10 impregnated with themonomer and the volatile solvent in such manner, subsequently, thevolatile solvent can be vaporize.

By the chemical polymerization between the oxidant used for theimpregnation of the capacitor element 10 and the monomer with which thecapacitor element 10 is uniformly impregnated, solid electrolytic layer5 comprising a dense and uniform poly(ethylenedioxythiophene) can beformed inside the wound capacitor element 10.

Because the duration of the thermal treatment is short, the productivityis great. Because methanol, ethanol and acetone with economy andrelatively ready handleability are used, the productivity can be moreimproved.

4.1 Production Process

The positive electrode foil 1 and the negative electrode foil 2 arewound through the medium of separator 3 comprising a nonwoven fabriccomposed chiefly of the vinylon fiber described above, to fabricatecapacitor element 10 as shown in FIG. 1. Herein, the capacitor element10 is with a nominal voltage of 20 WV and a nominal static capacity of10 μF.

The capacitor element 10 is then subjected to a process of forming themodified solid electrolytic layer. First, the capacitor element isimpregnated with a monomer solution of a mixture of3,4-ethylenedioxythiophene and a volatile solvent at a ratio of 1:1 to1:3 in volume. More specifically, the capacitor element 10 isimpregnated with 3,4-ethylenedioxythiophene and a volatile solvent. Bythermally treating the capacitor element 10 impregnated with the monomersolution in such manner, the volatile solvent is vaporized.

By thereafter allowing the capacitor element 10 to be impregnated withan oxidant solution, solid electrolytic layer 5 is formed by chemicalpolymerization between the oxidant solution and the monomer solutionpermeated into the separator 3. As the oxidant, herein, use is made offerric p-toluenesulfonate dissolved in butanol. In this case, the ratioof butanol and ferric p-toluenesulfonate is appropriately determined;one example is a 40 to 60% butanol solution of ferricp-toluenesulfonate. The blend ratio of 3,4-ethylenedioxythiophene andthe oxidant is preferably within a range of 1:3 to 1:6.

After the solid electrolytic layer 5 is formed on the separator 3interposed between the positive electrode foil 1 and the negativeelectrode foil 2 according to the processes, the exterior surface of thecapacitor element 10 is covered with an exterior resin, to recover asolid electrolytic capacitor.

4.2 Comparison of Processes of Forming Solid Electrolytic Layer

The process of forming the modified solid electrolytic layer isspecifically described below, together with specific comparison betweenthe examples of the solid electrolytic capacitor produced by the processand comparative examples.

Table 4 below shows five examples of Examples 9 to 13 of the process offorming the modified solid electrolytic layer and three comparativeexamples of Comparative Examples 4 to 6 of processes of formingcomparative solid electrolytic layer.

The individual examples in Table 4 are now described; in the inventiveExample 9, monomer impregnation process and thereafter thermal treatmentfor about several minutes are carried out prior to oxidant impregnation;in Examples 10 to 13, monomer impregnation process using volatilesolvents such as acetone and methanol and thereafter thermal treatmentfor about several minutes are carried out prior to oxidant impregnation,wherein the different types of the volatile solvents are used at variousvolume ratios thereof to the monomer.

In Comparative Examples 5 and 6, furthermore, the resulting capacitorelement is left to stand alone at ambient temperature with no use of anyvolatile solvent, wherein the time period from the monomer impregnationto the oxidant impregnation varies. In Comparative Example 4, stillfurthermore, the conventional technique for impregnation with a mixturesolution of a monomer and an oxidant is applied.

Comparative Example 4 impregnation with monomer/oxidant mixture solutionComparative Example 5 monomer impregnation - oxidant solutionimpregnation after 15 min Comparative Example 6 monomer impregnation -oxidant solution impregnation after 60 min Example 9 monomerimpregnation - thermal treatment -oxidant solution impregnation Example10 monomer/acetone = 1/1 solution impregnation - thermal treatment -oxidant solution impregnation Example 11 monomer/acetone = 1/2 solutionimpregnation - thermal treatment - oxidant solution impregnation Example12 monomer/methanol = 1/1 solution impregnation - thermal treatment -oxidant solution impregnation Example 13 monomer/methanol = 1/2 solutionimpregnation - thermal treatment - oxidant solution impregnation

4.3 Comparison Between Initial Performances

The initial performances of the solid electrolytic capacitors ofExamples 9 to 13 and Comparative Examples 4 to 6 were individuallydetermined. The results are shown below in Table 5.

As apparently shown in the results of Table 5, the solid electrolyticcapacitors of Examples 9 to 13 can exert more excellent performancevalues of static capacity, tan δ, and equal linear resistance (ESR) thanthose in Comparative Examples 4 to 6. Additionally, Comparative Examples5 and 6 show better values than Comparative Example 4; ComparativeExample 6 in particular requires a long time for monomer impregnation tooxidant impregnation with lower productivity than the productivity inExamples 9 to 13 with thermal treatment for a short duration for exampleabout several minutes.

TABLE 5 Static capacity ESR 100 120 Hz (μF) tan δ KHz (Ω) Comparative9.44 0.042 0.111 Example 4 Comparative 10.3 0.032 0.093 Example 5Comparative 10.3 0.024 0.092 Example 6 Example 9 10.1 0.036 0.077Example 10 10.4 0.033 0.076 Example 11 10.4 0.034 0.063 Example 12 10.20.034 0.060 Example 13 10.3 0.030 0.054

4.4 Modification Example

A specific process including the process of forming the modified solidelectrolytic layer is appropriately selected. Additionally, the processof forming the solid electrolytic layer is not limited to those ofExamples 9 to 13. The types of the volatile solvent may appropriately beselected, while the volume ratio of the monomer and the volatile solventis also appropriately selected within a range of 1:1 to 1:3. In Examples9 to 13, the capacitor element was impregnated with a monomer solutionof a mixture of the monomer and the volatile solvent, but the capacitorelement may satisfactorily be impregnated separately with the monomerand with the volatile solvent. In this case, the capacitor element canbe impregnated with a monomer solution with less compositional change.Furthermore, the types of the oxidant and the solvent and the ratiothereof can be selected appropriately.

5. Other Embodiments

The present invention is not limited to the aforementioned embodimentsand examples. The invention can be carried out in various modificationexamples within the scope of the invention.

Because the modified separator is individually used in the embodimentsrelating to the process using the oxidant at the modified concentrationsand the embodiments relating to the process including the process offorming the modified solid electrolytic layer, for example, the effectsof the modified separator can be brought about. The process includingthe process of forming the modified solid electrolytic layer by usingthe oxidant at the modified concentrations, can bring more excellentresults. More specifically, the process including the process of formingthe modified solid electrolytic layer by using the modified separatorand the oxidant at the modified concentrations can bring aboutsynergistic effects of these modifications in separator and oxidantconcentration. However, any one of the modifications can still bringabout the effect corresponding thereto.

Industrial Applicability

As has been described above, in accordance with the invention, anonwoven fabric composed chiefly of a synthetic fiber is used as themodified separator; and the nonwoven fabric comprises vinylon fiber, orvinylon fiber mixed with glass fiber, polyester fiber, nylon fiber,rayon fiber or paper fiber.

Such separator never reacts chemically with such oxidant and is misciblewith the solvent of the oxidant, although the separator is at athickness almost equal to the 40-μm thickness of a separator comprisingmanila paper for use in conventional electrolytic capacitors. Hence, theseparator never deteriorates the permeability of the monomer and theoxidant for use in impregnation, to improve the volume efficiency of theresulting capacitor element, so that the resulting solid electrolyticcapacitor is of a small size or with a large capacity.

By dipping the capacitor element with such rolled separator in water at80° C. to 100° C. for one to 10 minutes to dissolve and remove thebinder in the separator or by subsequently drying the resultingcapacitor element at 80° C. to 120° C., the binder in the separator canbe dissolved and removed, whereby the decrease of the static capacitydue to the presence of the binder can be prevented.

In accordance with the invention, an oxidant at a modified concentrationabove 40% by weight to a solvent is used, to form a dense and uniformsolid electrolytic layer inside the capacitor element, so that theresulting electrolytic capacitor can acquire excellent ESRcharacteristic.

In accordance with the invention, furthermore, a process of forming themodified solid electrolytic layer comprises allowing the preliminarilymonomer-impregnated capacitor element to be impregnated with an oxidant,to form a dense and uniform solid electrolytic layer inside the woundcapacitor element, so that the resulting electrolytic capacitor canprocure excellent electrical performance and a larger static capacity.

Typically, the inventive process comprises allowing the capacitorelement to be impregnated with a monomer solution of a mixture of amonomer and a volatile solvent, thermally treating the capacitorelement, and allowing the capacitor element to be impregnated with anoxidant solution, whereby a solid electrolytic layer of higher qualitycan be formed inside the wound capacitor element, so that the resultingelectrolytic capacitor can procure more excellent electricalperformance. The process is highly productive and very practical,particularly because economical and readily handleable volatile solventscan be used at the process and because the thermal treatment therein iscompleted in only several minutes.

What is claimed is:
 1. A solid electrolytic capacitor comprising acapacitor element fabricated by winding an anode electrode foil and acathode electrode foil through the medium of a separator, the capacitorelement being impregnated with 3,4-ethylenedioxythiophene and an oxidantto form poly(ethylenedioxythiophene) by chemical polymerization, saidcapacitor comprising: a nonwoven fabric being composed chiefly of asynthetic fiber and used as the separator.
 2. A solid electrolyticcapacitor according to claim 1, wherein the separator comprises anonwoven fabric composed of vinylon fiber, or vinylon fiber mixed withglass fiber, polyester fiber, nylon fiber, rayon fiber or paper fiber.3. A process for producing a solid electrolytic capacitor comprising thesteps of: winding an anode electrode foil and a cathode electrode foilthrough the medium of a separator to fabricate a capacitor element; andallowing the fabricated capacitor element to be impregnated with3,4-ethylenedioxythiophene and an oxidant to formpoly(ethylenedioxythiophene) by chemical polymerization, said processfurther comprising: in the foil winding step, a nonwoven fabric composedchiefly of a synthetic fiber being used as the separator; and the stepsof: dissolving and removing a binder in the separator, by dipping thecapacitor element wound by using the separator in water at 80° C. to100° C. for one to 10 minutes; and after the binder dissolving andremoving step, for the binder-removed capacitor element, theimpregnation step with the 3,4-ethylenedioxythiophene and the oxidant isperformed.
 4. A process for producing a solid electrolytic capacitoraccording to claim 3, wherein the separator comprises a nonwoven fabriccomposed of vinylon fiber, or vinylon fiber mixed with glass fiber,polyester fiber, nylon fiber, rayon fiber or paper fiber.
 5. A processfor producing a solid electrolytic capacitor according to claim 3, saidprocess further comprising the steps of: after the binder dissolving andremoving step, drying the capacitor element at 80° C. to 120° C.; andafter the drying step of the capacitor element, for the dried capacitorelement, the impregnation step with the 3,4-ethylenedioxythiophene andthe oxidant is performed.
 6. A process for producing a solidelectrolytic capacitor according to claim 5, wherein the separatorcomprises a nonwoven fabric composed of vinylon fiber, or vinylon fibermixed with glass fiber, polyester fiber, nylon fiber, rayon fiber orpaper fiber.
 7. A process for producing a solid electrolytic capacitoraccording to claim 5, wherein a series of the steps composed of thebinder dissolving and removing step and the drying step of the capacitorelement is repeated at least two times.
 8. A process for producing asolid electrolytic capacitor comprising the steps of: winding an anodeelectrode foil and a cathode electrode foil through the medium of aseparator to fabricate a capacitor element; and allowing the fabricatedcapacitor element to be impregnated with 3,4-ethylenedioxythiophene andan oxidant to form poly(ethylenedioxythiophene) by chemicalpolymerization, said process further comprising: in the impregnationstep of the capacitor element, an oxidant at a concentration above 40%by weight to a solvent being used as the oxidant.
 9. A process forproducing a solid electrolytic capacitor according to claim 8, whereinthe solvent is butanol.
 10. A process for producing a solid electrolyticcapacitor according to claim 8, wherein the oxidant is selected from thegroup consisting of ferric p-toluenesulfate, ferricdodecylbenzenesulfonate and ferric chloride.
 11. A process for producinga solid electrolytic capacitor comprising the steps of: winding an anodeelectrode foil and a cathode electrode foil through the medium of aseparator to fabricate a capacitor element; and allowing the fabricatedcapacitor element to be impregnated with 3,4-ethylenedioxythiophene andan oxidant to form poly(ethylenedioxythiophene) by chemicalpolymerization, said process further comprising: the impregnation stepincluding the steps of: allowing the capacitor element to be impregnatedwith a monomer 3,4-ethylenedioxythiophene; and after the monomerimpregnation step, allowing the resulting capacitor element to beimpregnated with an oxidant.
 12. A process for producing a solidelectrolytic capacitor comprising the steps of: winding an anodeelectrode foil and a cathode electrode foil through the medium of aseparator to fabricate a capacitor element; and allowing the fabricatedcapacitor element to be impregnated with 3,4-ethylenedioxythiophene andan oxidant to form poly(ethylenedioxythiophene) by chemicalpolymerization, said process further comprising: the impregnation stepincluding the steps of: allowing the capacitor element to be impregnatedwith a monomer solution of a mixture of 3,4-ethylenedioxythiophene and avolatile solvent; after the monomer solution impregnation step,subjecting the resulting capacitor element to thermal treatment; andafter the thermal treatment step, allowing the capacitor element to beimpregnated with an oxidant solution.
 13. A process for producing asolid electrolytic capacitor according to claim 12, wherein the volatilesolvent is a material selected from the group consisting ofhydrocarbons, ethers, esters, ketones, alcohols and nitrogen compounds.14. A process for producing a solid electrolytic capacitor according toclaim 13, wherein the volatile solvent is a material selected from thegroup consisting of methanol, ethanol and acetone.
 15. A process forproducing a solid electrolytic capacitor according to claim 12, wherein,in the monomer solution impregnation step, the3,4-ethylenedioxythiophene and the volatile solvent are separately usedfor impregnation.
 16. A process for producing a solid electrolyticcapacitor according to claim 12, wherein, in the monomer solutionimpregnation step, a monomer solution of a mixture of the3,4-ethylenedioxythiophene and the volatile solvent at a volume ratio of1:1 to 1:3.